Here's a breakdown of How I Met Your Mother's Slap Bet, including the rules and the circumstances behind Marshall's (Jason Segel) eight slaps and an explanation highlighting the rules behind the bet. Barney (Neil Patrick Harris) was the other figure involved in the bet and, eventually, the target of the slaps.
How I Met Your Mother had some of the best long-running gags in TV history. Interventions, Barney's Playbook, and the doppelgangers were just a few of the jokes that came back again and again in the series. One of the best jokes, however, was Robin's (Cobie Smulders) past alter ego, Robin Sparkles. She tried to keep her Canadian pop star identity a secret but Marshall and Barney's persistence uncovered the hidden gem. That revelation also spawned another long-running gag with the Slap Bet.
The gang tried to figure out what kind of secret Robin was keeping after she refused to go to malls. Marshall and Barney agreed to a Slap Bet regarding the secret with the loser receiving a hard slap in the face by the one who accurately predicted what Robin was hiding. Barney thought Robin had a past in pornography and when he first uncovered the Robin Sparkles video, he believed he won the bet and slapped Marshall. But it turned out that the video wasn't porn, it was a music video of Robin from when she was a pop star in the '80s. Therefore, Marshall won the bet.
Lily (Alyson Hannigan), the appointed Slap Bet Commissioner, allowed Barney to pick 10 consecutive slaps or five slaps at any moment Marshall decides for eternity as punishment for his premature slap. Barney chose the five-slap option and this is how they played out over the rest of How I Met Your Mother:
- Slap One - The first slap occurred in season 2, episode 9 which was appropriately titled "Slap Bet." Shortly after Marshall officially won the bet, he waited for Barney to let his guard down and suddenly slapped him across the face.
- Slap Two - The next slap comes seven episodes later in "Stuff" when Barney decided to put on a one-man show after Lily's excruciatingly bad play. As Barney started to tap dance heading into his grand finale, Marshall popped up from his seat and slapped Barney on stage to make him stop.
- Slap Three - During the Thanksgiving episode in season 3, "Slapsgiving", Marshall terrorized Barney with a countdown to the third slap. Lily proclaimed that no slaps are allowed on the holiday but due to Barney's taunting, she changed her mind. Marshall gave Barney a fierce slap, creating a holiday better known as Slapsgiving.
- Slap Four - The ninth episode in season 5 was called "Slapsgiving 2: The Revenge of the Slap." Marshall decided to give up his holiday slap to either Ted (Josh Radnor) or Robin. They each passed it around to the attendees at the holiday gathering. In the end, Marshall explained he did so to bring everyone together with the urge to slap Barney. He then took one for the team and gave Barney his fourth slap.
- Slap Five - The next slap didn't occur until the middle of season 7 in "Disaster Averted", but it also cane with a change the slap bet rules...
Lily's role as Slap Bet Commissioner was to oversee the slaps and to deem them fair and appropriate. She instituted a set of rules including the fact that Marshall could not wear any rings or gloves on his hands when he hit Barney. She was in charge of handling any kind of rulings or discrepancies between the two parties. Lily also had the final word which came in handy during slap five. It was supposed to be the last slap but she extended the punishment to three more slaps.
- Slap Six - Barney was required to wear the Ducky Tie for the rest of the year. When he refused, Marshall and Lily agreed that it was only fair to extend his slap count. Marshall then handed out two slaps.
- Slap Seven - In season 9's "Slapsgiving 3: Slappointment in Slapmarra", Marshall teased that he's been working towards the "Slap of a Million Exploding Suns." After Marshall got the job done, Boyz II Men appeared to sing "You Just Got Slapped."
- Slap Eight - The final slap came during the penultimate episode of How I Met Your Mother, "The End of the Aisle". As Barney was waiting for Robin to walk down the aisle, Marshall gave him the last slap which finally knocked sense into him.