Ludogorets began with a two-goal lead, but Plzen made a game out of it early, before giving up two goals, losing 4-2 on aggregate.
The quarterfinalists from two seasons ago make it to the group stage with ease by beating Villarreal 3-1 on aggregate.
Celtic allowed two goals to Hapoel Be'er Sheva but the Scottish side still qualified for the UCL group stages on aggregate, 5-4.
Roma's Champions League campaign is over before it began, losing 3-0 to Porto and earning two red cards in the process.
Celtic's Craig Gordon has the ball knocked out of his hands by Saidy Janko, which was hit home by Ben Bitton.
Robbie Benson scored what could be the goal of his life, yet Dundalk fell 3-1 on aggregate to Legia.