LEONARD: Previously on
The Big Bang Theory...
The Big Bang Theory Season 10, Episode 23
♪ That all started
with a big BANG ♪
The Big Bang Theory Season 5, Episode 6
♪ Bang!♪
The Big Bang Theory Season 12, Episode 5
♪ Bang! ♪
The Big Bang Theory Season 7, Episode 3
♪ That all started
with a big BANG ♪
The Big Bang Theory Season 7, Episode 23