Topher Grace dropped by Global‘s “The Late Show with Stephen Colbert” on Thursday and revealed that he can do only one impression — but it’s a doozy.

Chatting with host Stephen Colbert, the former “That ’70s Show” star explained that whenever he and his wife happen to catch “Back to the Future” on TV he would find himself slipping into the vocal pattern and mannerisms of Michael J. Fox’s Marty McFly.

Starting with a quick facial impression, Grace followed up with a pitch-perfect imitation of one of Fox’s more memorable lines from the 1985 classic.

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“Doc,” he declared in full-on Marty McFly mode, “are you telling me you built a time machine out of a DeLorean?”

Later in the interview, Grace told Colbert that he’s embraced a new hobby: re-editing movies. One of his projects, he revealed, was re-editing Peter Jackson’s “Hobbit” trilogy into one film.

That got Colbert’s attention, as the “Lord of the Rings” uber-fan actually made a brief cameo in 2013’s “The Desolation of Smaug”.

“How long is your movie?” Colbert asked, pointing out that the combined running time of Jackson’s three films is nine hours.

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“That is not the question you want to ask me,” remarked Grace, as Colbert reminded his guest he played “the master of the spy network in Lake Town, a crucial, critical character” in the film.

“It’s maybe 15 seconds, right?” said Grace of Colbert’s cameo.

“Is it still in your f**kng movie or not?” the suddenly steamed host asked.

“I’m so sorry, I’m so sorry,” Grace admitted. “We had to cut it. I’m so sorry. Here’s what happened, Stephen: When you get in the editing room, and you have all these choices–”

Quipped Colbert: “That’s all we have time for, Topher, I’m afraid.”

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“The Late Show With Stephen Colbert” airs weeknights at 11:35 p.m. ET on Global.