But who is Brett? And what will his arrival mean for his future with Whitney?
When Zack checks with Whitney that she’s happy for him to take the cruise ship job, she tells him to go for it, thinking it’s what he wants.
Later, Chelsea collars him and tells him Whitney doesn’t want him to leave and feeling pressured, an angry Zack tells her that he's only taking the job to make lots of money to support the baby.
After cooling down, Zack turns down the opportunity and whilst Whitney is thrilled, Zack’s happiness is short-lived after spotting an old acquaintance called Brett.
With Zack on a mission to convince Ravi to give him the Head Chef role and a pay rise at Walford East, Whitney acts as his cheerleader. But things go horribly wrong when Zack spots Brett and gets rattled by his presence.
Meanwhile, Sharon interviews Brett for a PT job at Crunch Time, only for Zack to warn her off employing him. Who is Brett?
Later, during a heated confrontation, Zack tries to scare Brett into leaving Walford but instead drops a bombshell that turns Zack’s world upside down.
With Zack in total denial, Sharon’s concerned to see Brett hanging about and Zack promises that he's changed his ways.
Whilst Whitney is pleased to hear that Zack has convinced Ravi to give him a second chance, she's worried by his lack of enthusiasm about the ultrasound.
Later, a broken Zack goes to the club to drown his sorrows and ends up in a drunken row with Whitney. As he continues to drink, Zack smashes a glass in a fit of rage, cuts his hand and breaks down.
Knowing that something is wrong with Zack, Sharon tells him she’s here for him whenever he’s ready to talk but Chelsea isn't buying his change of heart over the baby and is determined to talk sense into him.
Alone in the Walford East kitchen, Zack has an accident and breaks down and sobs.
EastEnders continues Mondays to Thursdays at 7.30pm on BBC One