Lucy Liu as Joan
Joan (Lucy Liu) finds out about her estranged father’s death in Season 6, episode 2 of CBS’ “Elementary.” CBS

Someone from Joan’s past passes away in the next episode of “Elementary.”

According to the synopsis for Season 6, episode 2 of the CBS series, Joan’s (Lucy Liu) estranged biological dad dies, and she and her half-sibling Lin (Samantha Quan) have conflicting reactions to his death.

In a sneak peek from the hour, Lin meets up with Joan to hand her a letter. “I got a call this morning from the shelter where dad was staying,” Lin tells her sister. “They said they had some of his personal effects, and they were wondering if I wanted them.”

Lin said that he went to the shelter hoping to find something her dad kept to remember her by. “I thought I might find an old picture of me and my mom, or some grade school art project I gave him,” Lin tells Joan. “[But I only found] his water bottle, a watch that didn’t work, an old dictionary I’m pretty sure he dug out of a dumpster and [a letter for you].”

When Joan asks if their dad wrote them letters, Lin notes that the surgeon-turned-detective was the only one who got one. “It’s OK, seriously,” Lin says, “at least he remembered one of us, right?”

While the content of the letter is unclear, executive producer Rob Doherty revealed to TVLine last October that Joan will learn that her estranged dad “made certain assessments regarding the decisions Joan has made as a professional.” “It’s going to give her cause to look at choices she’s made and choices that she has right in front of her,” Doherty said. “Might she be a more complete person if she weren’t in a partnership with Sherlock Holmes (Jonny Lee Miller)?”

Elsewhere in the next episode, Sherlock and Joan find themselves on the hunt for a stolen plutonium shipment they fear will be used to make a dirty bomb after a department of energy inspector is killed.

In another sneak peek from the installment, Captain Gregson says that the department of energy inspector Rohan Giri (Alok Tewari) was “hit in the pelvis by a billion bolts of electricity,” causing his death.

But what puzzling Sherlock and Joan is that natural lightning would never have struck a standing man in his lower torso.

“His scorch marks would start on his head or upper body,” Joan notes, as they examine Rohan’s body.

“Precisely,” Sherlock says. “But this bolt of lightning did not travel vertically. They travelled parallel to the ground like a bullet fired from a gun.”

When Joan adds that there were no puncture marks that would indicate that Rohan was hit by a taser, and his burn patterns don’t match a stun gun or a cattle prod, Sherlock arrives at a conclusion that the victim was killed using an unknown “experimental” weapon.

After Marcus (John Michael Hill) finds out that Rohan and her physicist wife had a history of domestic disputes and accusations of infidelity on both sides, the team starts to consider the victim’s wife as the primary suspect.

“Elementary” Season 6, episode 2, titled “Once You’ve Ruled Out God,” airs on Monday, May 7 at 10 p.m. EDT on CBS.