In the episode, the Cerritos is expecting crew to visit from another ship, and Boimler, who’s eagerly anticipating their arrival, agrees to help Rutherford with a transporter, which he’s been asked to make work a bit faster.
The first run goes swimmingly, but they get cocky, with Boimler asking to be run through the transporter again and Rutherford gleefully complying, thus leading to this episode’s “space accident” which leaves Boimler slightly out of phase with spacetime, blue, transparent, and walking around with that very familiar transporter sound firmly attached to his newly changed person.