Thallium is a highly toxic tasteless, odourless and water-soluble metal that can be absorbed through the skin, inhaled or ingested. Due to the rarity of thallium toxicity, it is frequently misdiagnosed or the diagnosis is delayed. We report a 41-year-old male landscaper admitted for acute polyneuropathy and abdominal pain. He was treated for suspected Guillain-Barré syndrome and later autoimmune encephalopathy. However, over the next 42 days, he developed worsening muscle weakness, delirium and alopecia, and was diagnosed with thallium toxicity. After combining Prussian blue, activated charcoal and continuous venovenous haemofiltration, he improved though with neuropsychiatric and neuromuscular sequelae. We highlight the need to manage information disclosure properly and to preserve evidence, when the source of a toxin is unclear.