Modern technical systems are increasingly situated in changing and thus variable environments, characterised by heterogeneous components that may not initially be able to perceive other components around them; the ability to overcome such problems despite lack of knowledge is essential for avoiding catastrophes. In nature, organisms can immediately and temporarily adapt their behaviour to novel stimuli by exhibiting behavioural plasticity; this can be achieved with neuromodulation, a biological process that modulates synaptic activity in the brain. We explore how plasticity via neuromodulation affects the goal-achievement of evolved neural controllers for artificial agents in variable environments. As variability can arise from the actions of others, we show that plasticity is beneficial in variable environments; not only this, but we demonstrate that the benefit of plasticity increases with variability, as agents can temporarily change their phenotype within their lifetime. Further, we show that cooperative behaviour can emerge between plastic agents that cannot perceive one another in highly variable environments.