Season 9: Episode 11 - The Rehearsal Dinner

The Rehearsal Dinner: Barney becomes convinced that Robin is planning a surprise rehearsal dinner at a Laser Tag arena. This leads Barney and Robin to examine the general culture of prank-playing (Barney, with the proposal, Robin with the bachelor party) that has defined their engagement so far, and whether that culture will continue as they transition into marriage. Ted, meanwhile, attempts to learn piano so he can perform dressed as Liberace at Barney and Robin's wedding. Lily struggles with the fact she can't keep a secret.