[Warning: Spoilers for 'Ace Reporter' ahead]

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Supergirl returned to The CW this week with 'Ace Reporter', an episode that returned to the show's roots by pitting Supergirl (Melissa Benoist) against a big bad borrowed from the comics. The episode also focused on Kara's relationship with Lena Luthor (Katie McGrath), and Lena's fear of becoming as ruthless as her mother. The episode ends with Supergirl getting the bad guy (obviously), and with her getting a little bonus, too: Kara has her job back at CatCo.

'Ace Reporter' also included a side plot about the friendship between Winn (Jeremy Jordan) and James (Mehcad Brooks), but one character didn't get a whole lot of screen time this week: Alex Danvers (Chyler Leigh). That's all going to change next week, however, in an episode all about Kara's sister, and a new promo reveals just how much danger Alex is going to be in.

The promo is a quick teaser for next week's episode 'Alex'. At 21 seconds long, there isn't enough time to show a whole lot. The promo focuses on the central plot of the episode, starting with Kara getting a phone call at work. A voice tells her that she has 36 hours to break a criminal out of prison, or her sister dies, and we see Alex in a glass box, slowly filling with water.

Supergirl Homecoming Alex Kara Danvers

Fans have previously been told that this is going to be a big episode for Kara and Maggie (Floriana Lima), Alex's girlfriend. The two have to work together to save Alex, and this may lead to the 'big surprise' that the creators have been teasing for Alex and Maggie this season.

It's interesting to note that whoever calls Kara refers to her as 'Supergirl', yet calls her on Kara's phone -- which means that they know Kara's true identity. This could be particularly dangerous, or it could mean that someone who already knows her identity is behind it all. It certainly looks like this is going to be a particularly tense episode, using the classic 'timed death machine' to put the pressure on Supergirl to find Alex and save her. This is classic superhero stuff, and it's definitely going to be exciting to see how Kara reacts.

It's not yet clear who the criminal is that Supergirl is expected to get released, or how her adversary knows her identity, but we only have to wait one week to find out.

Supergirl continues Monday, May 1 with ‘Alex’ at 8pm on The CW.

Next: Supergirl: Ace Reporter Review & Discussion

Source: The CW