Update (June 20): Cartoon Network released the full trailer — with an alternate ending! — online.

The first full trailer for long-gestating Cartoon Network series Infinity Train is here — except you’ll have to work to find it.

An interactive website prompts you to click a glowing green “Enter” on an otherwise dark screen, spacey synth music echoing as the screen loads and presents a dreamy music-themed starscape. To unlock the trailer, you must play the right sequence of notes on the piano (spoiler: it’s the high D, the B right below, the G above, and F#). Here’s what it sounds like:

Infinity Train originally started as an eight-minute cartoon by Regular Show writer and storyboard artist Owen Dennis. The 2016 short gained over 1 million views on YouTube within the first month, and a petition to officially launch the series amassed 57,000 signatures before the series itself was greenlit.

The short — now the pilot episode — finds a girl named Tulip trapped on a never-ending train, in which each car opens up into its own pocket universe. A teaser for the show premiered in 2018, setting the ambience for the show, with Tulip in a wasteland and a glowing number on her hand.

This new trailer reveals more of the wacky sub-dimensions, the kooky characters that accompany Tulip (including a spherical robot and a talking corgi king), and an ominous, tentacled robot bent on preventing Tulip from disembarking the train.

Infinity Train is set to premiere later in 2019.