A study to learn how bimekizumab works in people with moderate to severe plaque psoriasis.

Lab values

• Cohort A and Cohort B - Study participant has a form of PSO other than plaque type (eg, pustular, erythrodermic and guttate PSO, or drug induced PSO) - Study participant has an active infection or history of infection(s) as follows: a) Any active systemic infection within 14 days prior to Baseline b) A serious infection, defined as requiring hospitalization or intravenous anti-infective(s) within 2 months prior to the Baseline Visit c) A history of opportunistic, recurrent, or chronic infections that, in the opinion of the investigator, might cause this study to be detrimental to the study participant - At investigator’s discretion, study participant with chronic (medically controlled) viral hepatitis B or C or human immunodeficiency virus (HIV) infection, or history of hepatitis B. - Study participant has any of the following: a) Known active tuberculosis (TB) disease. b) History of active TB involving any organ system unless adequately treated c) High risk of acquiring TB infection - Study participant has a verified diagnosis of inflammatory conditions other than PSO or PsA, including but not limited to rheumatoid arthritis (RA), sarcoidosis, inflammatory bowel diseases (IBD), or systemic lupus erythematosus. Note: Study participants with a diagnosis of IBD are allowed if they have no active symptomatic disease at Screening or Baseline - Study participant has any active malignancy or history of malignancy within 5 years prior to the Screening Visit EXCEPT treated and considered cured cutaneous squamous or basal cell carcinoma, or in situ cervical cancer - Study participant has any medical or psychiatric condition that, in the opinion of the investigator, could jeopardize or would compromise the study participant’s ability to participate in this study - Study participant has a known hypersensitivity to any components of the IMP as stated in this protocol - Study participant has a history of primary failure to any biologic (ie, no response within the first 12 weeks of treatment) - Study participant has laboratory abnormalities at Screening - Study participant has a current history of alcohol or drug use disorder, as defined in Diagnostic and Statistical Manual of Mental Disorders (DSM) V, within the previous 6 months prior to Screening, as evaluated by the investigator based on medical history, and/or site interview

• Cohort A and Cohort B - Study participant must be at least 18 years of age inclusive at the time of signing the Informed Consent Form (ICF) - Study participant must have: a) Cohort A and Cohort B: Plaque psoriasis (PSO) diagnosed for at least 6 months prior to the Screening Visit b) Cohort B only: In addition to the criteria specified above, study participant has a documented diagnosis of adult-onset psoriatic arthritis (PsA) and meets the CASPAR classification criteria for at least 6 months prior to Screening for active PsA and must have ≥1 tender joint count (TJC) out of 68 and ≥1 swollen joint count (SJC) out of 66 at Screening or up to 3 months before Screening (documented evidence) - Study participant must have Psoriasis Area and Severity Index (PASI) score ≥12 and body surface area (BSA) affected by PSO ≥10% and Investigator’s Global Assessment (IGA) score ≥3 on a 5 point scale - Study participant must be a candidate for systemic PSO therapy and/or phototherapy - Study participant agrees not to change their usual sun exposure during the course of the study and to use ultraviolet A/ultraviolet B sunscreens if unavoidable exposure occurs - Study participant has body weight <120 kg - A female study participant is eligible to participate if they are not pregnant, not breastfeeding, and at least 1 of the following conditions applies: Not a woman of childbearing potential (WOCBP) OR A WOCBP who agrees to follow the contraceptive guidance during the Run In Treatment Period, the Randomized Treatment Extension Period, the Treatment Extension Period, the Escape Treatment Period, and for 17 weeks after the final dose of investigational medicinal product (IMP)