Season 1: The 1953 H.G. Tannhaus (Arnd Klawitter) and 1986 H.G. Tannhaus (Christian Steyer) — sweater-vest-loving scientist and clockmaker — equally (and inadvertently) play their part in keeping the wormhole below the town open by helping to invent a special time-travelling device.
Season 2: H.G. Tannhaus appears primarily in the form of his books and photographs that are in his granddaughter Charlotte Doppler's attic. One of those old black-and-white photographs shows a group of men that include Adam and Noah. When Charlotte's daughter Elisabeth recognises Noah in the picture, Charlotte and her husband Peter begin trying to figure out what exactly is going on in Winden — and exactly who Charlotte's real parents are (spoiler alert: its Noah and her own daughter, Elisabeth).
Season 3:
- Adam's World: In the '80s, we see Tannhaus give Charlotte more of her backstory, and learn that his son, daughter-in-law, and granddaughter (also named Charlotte) were killed in a car crash in the '70s.
- Eva's World: H.G. isn't shown in this world, though we'd be lying if we didn't say that even we were a little confused as to whether his aforementioned conversations with Charlotte were in Adam's world or Eva's.
- Origin World: It's discovered in the final episode that H.G. is the reason Adam and Eva's worlds were created in the first place, and we see him from 1974 through 1986 building the time machine that will destroy the origin world and ultimately create Adam and Eva's. Once Jonas and Martha prevent H.G.'s son, Merrick, from dying in 1971, Merrick returns to H.G.'s home with his wife and daughter, Charlotte. We last see H.G. holding his granddaughter in his clock shop, with no reason at all to ever create time travel.