Season 5: Episode 85: Barely Legal
Meg can't find a date to her junior prom, and even though he is dating Jillian, Brian ends up going with Meg. At the prom, Brian gets drunk to make the night pass, but ends up making out with Meg. Meg begins to think of Brian as her boyfriend and becomes obsessed with spending every moment with him. Meanwhile, Peter, Cleveland and Quagmire join the police force to help Joe, and end up saving Brian.