In 2015, the US scientist, artist and self-described ‘biohacker’ Josiah Zayner undertook a controversial project to help resolve his lifelong gastrointestinal issues. The plan was to replace the vast colonies of microbiota on and inside his body via transplants from a healthy donor – and then document the proceedings. Although an accomplished biologist with a PhD in biophysics and two years as a NASA researcher under his belt, Zayner’s endeavour was frowned upon by much of the scientific community, with critics condemning the project for operating outside the normal boundaries of bioethics. Especially controversial was Zayner’s plan to self-administer a faecal transplant – a risky procedure usually reserved for potentially fatal conditions. In their documentary Gut Hack, the filmmakers Mario Furloni and Kate McLean follow Zayner’s fascinating, radical and not-for-the-squeamish quest for relief. In so doing, they also confront deeper issues of ethics and autonomy at the core of contemporary science.