The four boys have finished their A-levels (after the end of the Inbetweeners TV series). Simon's move to Swansea was called off and he's now going out with Carlie, although she announces she wants to break off with him, which leaves him devastated and sobbing. Neil is still seeing Nicole, the woman from the fish counter at Tesco. Jay and Will are still alone and dorky, but the death of Jay's grandfather has left him with some inherited spending money. After a typically encouraging speech from the Head of Sixth, they leave school for the last time (with Donovan giving Will a final wedgie). They decide to go on a summer holiday to Greece together to celebrate the end of their school days and help Simon get over Carlie.
Arriving in Greece they move into their lousy hotel and head off to find the night-life. When a police car drives past, Jay dives out of the way, saying that foreign police can't be trusted and that "he has a 20 euro note up his ass for a bribe just in case". The boys end up being sent by a canvasser into a club called Marco's which turns out to be deserted. Hanging out there for a while, four British girls - who they recognize from the flight they were on - arrive at the club too, and they pair off together. However, Simon spends all of his time talking to Anne about how much he misses Carlie; Neil walks away from his girl to sexually "entertain" older women in the club; Jay pairs off with a fat girl and drinks heavily while talking to her awkwardly; and Will's girl, Sophie, turns out to already have a Greek boyfriend called Nikos. In spite of this they agree to meet up again another time, and are told that there's an all-day boat party in a couple of days, but the tickets are sold out.
As they leave the club, Simon thinks he sees Carlie across the street. The others point out he's on the rebound, but Neil adds that it's possible it is Carlie because she is in the same city - Neil consulted Carlie on where would be a cool place to go for summer. Simon rushes over to talk to Carlie, and she mentions she's going to the boat party before Simon is run over by a quad bike driven by Carlie's holiday rep, who it seems she has eyes for.
The following day, all four boys go to the pool at the girls' hotel to meet up with them again. Will hangs out with Sophie, while Simon pushes Anna into leaving to go and try and find some extra boat party tickets from her holiday company. Jay and Neil try to impress the other girls around the pool, but Jay is humiliated by a young boy playing tricks on him. Jay eventually catches the boy and throws him in the pool, at which point it turns out the boy cannot swim and has to be rescued from the pool. With this plus Will offending the disabled daughter of the group whose sunloungers the boys sat on, all four of them are thrown out of the pool area. Simon is furious with Jay for having gotten him thrown out before Anna could get back with the tickets, and the two fight on the street before being separated by Neil and Will and heading off in two separate groups.
Simon and Will return to the hotel where Simon decides to sell the spare clothes he brought with him in an attempt to raise money to buy a ticket second-hand. Carlie's rep offers to take the clothes to a buyer, but instead just leaves with them and never returns with the money. Simon is left wearing Will's t-shirt as a pair of pants. The two head to Marco's again to get drunk. Meanwhile, Jay and Neil are heading off to explore other clubs. Jay reveals that he actually already bought second-hand boat party tickets for all of them, but tears them up in anger at how he was treated by Simon. Jay and Neil try to get into a group of cool boys at one of the busier clubs, but are only teased and humiliated and threatened with violence. Jay breaks down in tears and the two return to Marco's where the four of them make up.
At Marco's the boys are getting drunk when the four girls arrive again and invite the four of them to go skinny-dipping with them on the beach. Simon and Anna wade out into the water but Simon immediately turns away when he sees Carlie passing on the shore. Jay talks with the fat girl but is embarrassed by other boys on the beach making fun of her weight, and realizing he doesn't want to be seen with her the girl walks away. Will is shy of undressing and so Sophie undresses him, also throwing away his glasses, which he immediately has to go and fetch. In doing so, he accidentally stumbles over a couple making love on the beach. When Sophie comes to find Will, she finds that the man in the couple is Nikos, and screams at him and Will before leaving abruptly.
All four boys return to the hotel feeling down, and Neil tries to cheer Simon up. The four of them decide to forget and what's happened so far and try to enjoy the holiday. On the day of the boat party, Neil has obtained a ticket from one of the older women, Sophie has tickets for herself and Will, and Jay has bought a ticket. Simon has no ticket and meets up with Anna, who offers to hang out with him by the hotel pool instead of going to the boat party even though she has a ticket. Simon is surprised by her willingness to do this, but unsure he wants to give up meeting with Carlie. Anna offers him her boat party ticket instead, if he really wants to go, and he abruptly snatches it from her and leaves her by the pool.
At the boat party, Sophie confesses she has feelings for Will, and Neil's girl tears him away from the one elderly woman present. Jay apologizes to the fat girl and they make out in the toilets on the boat. Simon meets up with Carlie and she seems to be keen to get back together, and they kiss several times, but he sees that she is still looking at the rep over his shoulder. Realizing how Anna feels, Simon asks one of the other boys to text Anna that he's coming to meet her, and with the rest of the party cheering him, jumps from the boat aiming to swim to the shore. But the swim is too long; Simon becomes exhausted and has to be taken to the shore by air-sea rescue, where Anna meets him in hospital. Meanwhile, Carlie's rep meets up with Jay and the fat girl as they come out of the toilet together, insults the fat girl, and agrees to keep quiet about what they've done if Jay will give him a note for him to snort drugs with. Jay gives him the 20 euro note from up his ass, and the rep is left walking around on the boat with a piece of poo lodged in his nostril. The credits roll over the boys flying home and then meeting up with the girls again at the airport in England.
Thanks Mark G!