Marvel fans were left confused when, in Avengers: Infinity War, the Hulk refused to come out and fight Thanos - but Avengers 4 will explain why. Mark Ruffalo has said that Thor: Ragnarok was the beginning of a "Hulk Trilogy" that continued in Infinity War and will conclude in Avengers 4, and that this arc has been planned since the early development of Ragnarok.

Hulk is a somewhat unusual character in the Marvel Cinematic Universe, since he was one of the most famous Marvel superheroes before it began, but (due to rights issues) hasn't appeared in his own solo movie since 2008's The Incredible Hulk. Moreover, since Bruce Banner was played by Edward Norton in that movie, many forget that Incredible Hulk is even canon within the MCU - though there have been numerous references to it since, and antagonist Thaddeus Ross (William Hurt) returned in Captain America: Civil War and Infinity War.

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Because he has only appeared in team-up movies and other superheroes' solo movies, Ruffalo's Bruce Banner hasn't enjoyed the kind of screen time and character development that, say, Tony Stark has. However, in between the alien invasions, Bruce and the Hulk have managed to eke out a character arc in the six years since The Avengers - including a romantic dalliance with Black Widow, accidentally co-creating a psychopathic robot with Tony, and flying off to an alien planet and becoming a celebrated gladiator. Let's take a look back at his two most recent movies, and what they might mean for Avengers 4.

Hulk's Three-Movie Arc Is Really Weird So Far

Thor and Bruce Banner Hulk in Thor Ragnarok

Hulk's trilogy began after he took off in the Quinjet at the end of Avengers: Age of Ultron - seemingly out of a desire to keep away from people he cared about (like Natasha Romanoff), so that he couldn't do any more harm. When Thor found him again on Sakaar in Thor: Ragnarok, Hulk had stayed big and green for two long years, had better control over his rage, and had even learned to speak in full (if somewhat abridged) sentences. A video message from Natasha stored on the Quinjet finally triggered Hulk's transformation back into Bruce, though the green giant fought it every step of the way, yelling, "No Banner!" When Bruce woke up, he had no memories of anything after transforming for the final climactic battle of Age of Ultron - asking Thor if they had managed to save Sokovia - which indicates that the Hulk alone was responsible for the decision to leave Earth.

After an awkward landing, Bruce transformed back into the Hulk to defeat Hela's giant wolf, Fenrir, at the end of Ragnarok. Then, when Infinity War picked up Thor and Hulk's story, Hulk was soundly beaten by Thanos and transported back to Earth by Heimdall. What's particularly intriguing about that last detail is the fact that Heimdall decided to transport Hulk, whom he barely knows, rather than save Thor's life - though one theory is that it has to do with the extreme toll that travelling via the Bifrost causes the traveller. Heimdall may have simply sent the one person (or, in this case, giant green monster) who has the best chance of surviving the journey. Still, it's an odd detail, and Hulk's story only gets odder from there.

When Thanos' minions arrive in New York, Bruce tries to team up with Doctor Strange, Wong, Spider-Man and Iron Man - only for Hulk to refuse to come out and play. In fact, beyond the opening scene, the Hulk is never seen again except for brief glimpses of him fighting Bruce's attempts to transform. Not to be left out of the fight, however, Bruce dons Tony Stark's Hulkbuster armor and uses it to battle the Outriders and the remaining members of the Black Order in Wakanda.

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The Avengers running towards the camera in an Infinity War trailer shot.

The big question, of course, is why Bruce was left to fight the biggest battle of his life on his own, when it was exactly the sort of situation that called for a good old-fashioned smashing. According to co-director Joe Russo, it's not (as some fans speculated) because Hulk was so rattled by his first fight with Thanos that he was too scared to come out for round two. Apparently, after years of being called upon to fight all of Bruce's battles for him, Hulk was simply rebelling:

"What makes him unique as a character is that there is a host body that is being fought over by two distinct personalities who hate each other. And both want control of the host body. So we thought an interesting direction to take him in is what if Banner, who typically uses the Hulk to solve crisis situations, what if the Hulk were no longer interested in solving those problems for Banner? So that the relationship is becoming increasingly dysfunctional... I think ultimately what it is, is that he's tired of playing hero to Bruce Banner."

Now, that's not the greatest explanation for Hulk's refusal to fight. If that were really the reason for it, then it seems like Infinity War should have found time for a line or two from Hulk telling Bruce that he was on his own this time. It also wouldn't be the first time that the Russos deliberately misled audiences in order to disguise a spoiler (they did so in the trailers for Infinity War, which showed clips Hulk fighting in Wakanda in all his green glory). Fortunately, Infinity War set things up so that we might get a better explanation in next year's Avengers 4; when the dust clears at the end of the movie, Bruce is one of the few superheroes left standing.

Page 2: How Will Hulk's Trilogy Conclude In Avengers 4?

Bruce Banner next to the Hulkbuster Gauntlet in Avengers Infinity War

Avengers 4 Is the Final Movie in the 'Hulk Trilogy'

Ruffalo's Bruce Banner has so far been in four MCU movies (five, if you count his appearance in Iron Man 3's post-credits scene), but the character is owned by Universal Studios and, as Ruffalo put it, "For some reason, they don’t know how to play well with Marvel. And they don’t want to make money." So, in lieu of making another standalone movie starring Ruffalo, Marvel Studios boss Kevin Feige decided to give Hulk a story arc that was spread across supporting and ensemble roles in three movies. Ruffalo explained:

"Basically, Kevin pulled me aside before this, and said, 'If we were going to do a standalone Hulk movie, what would it be?' And I said, 'I think it should be this, this, this, and this and this, and ends up like this.' And he's like, 'I love that. Why don't we do that in the next three movies, starting with Thor 3 and then we go into Avengers 3 and 4.' And I was like, 'That sounds great!' And so we are at the beginning of this arc."

Speaking to Fandango at the D23 Expo in 2017, Ruffalo said, "There's a huge change for Hulk and for Banner, and it doesn't get resolved in the first Infinity War, it kind of culminates in the second Infinity War." So, while Hulk's behavior in Infinity War was certainly weird, and the explanation given by Joe Russo doesn't completely make sense of it, it's one of many unanswered questions from that movie that will presumably be resolved in next year's sequel.

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Avengers 4 May Unite Banner and Hulk's Personalities

The Incredible Hulk - Green Eyes

Back when we first met the MCU's Bruce Banner in The Incredible Hulk (when he looked a little different), there was little indication that he considered his big green alter-ego to be an independent personality. He referred to and treated the Hulk like an unfortunate condition, and did his best to "cure" himself of it. At the end of the movie he appeared to have attained a level of control over the Hulk - first "aiming" it in order to take down Abomination, and then using meditation to voluntarily summon the change.

When MCU fans met Bruce again at the start of The Avengers, he was already referring to the Hulk as "the other guy" - setting up the dynamic that would eventually form the basis of the current Hulk trilogy: a push-and-pull between Banner and Hulk, with each fighting for control of the same body. Hulk effectively kept Bruce imprisoned for two years while on Sakaar, and in Infinity War he changed tactics and refused to come out at all. It seems clear that things are hurtling towards some kind of showdown in Avengers 4.

The outcome of this showdown could go a number of different ways. We could end up saying goodbye to both Bruce and the Hulk in some kind of heroic sacrifice, since Marvel is unable to make Hulk solo movies, and he's therefore one of the more "expendable" Avengers. If the Hulk has to go out, the two personalities finally finding harmony and working together would be a poignant way to do it. Alternatively, the movie may conclude with the two characters somehow separating entirely, as Ruffalo hinted at in an interview for Ragnarok:

"This is where it starts... we start to build out the separation between the Hulk and [Bruce] Banner. Now the Hulk can be his own character, and Banner can now start to be the person that he would be without having to be afraid of being excited all the time."

Our final theory is that Avengers 4 will conclude with the two personalities united as one: "Professor Hulk." Concept art for the movie seemed to suggest a somewhat more civilized Hulk, wearing an actual suit instead of just tattered rags, and it would make sense as a conclusion to the trilogy - and, really, a conclusion to Hulk's arc over the past ten years.

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